The field of the invention pertains to means for sensing the fluid pressure within a mold cavity and, in particular, to sensing the in-cavity fluid pressure exerted axially against the ejector pins or dummy pins of a plastic injection mold, compression mold, die cast mold or metal consolidation mold.
Available are load cells constructed of piezo-electric or electro-mechanical transducers sealed in steel bodies that are placed at the ends of the ejector pins remote from the mold cavity. The ejector pins rest on sensing elements atop or located in the top surface of the steel bodies.
A small cavity or oblong slot is milled into the mold ejector plate beneath the ejector pin for the placement of a sensor. Where the slot extends full width straight to the edge of the ejector plate, an oblong slide mold pressure sensor may be inserted and removed without disassembly of the mold and ejector plate. Where mold configuration does not permit a straight full width slot, a small channel for the electric wires to a button mold pressure sensor is provided. In the latter configuration, the mold and ejector plate must be disassembled to insert or remove the sensor.
Slide mold pressure sensors are exemplified by the D-M-E Standard SS-405C and SS-406C sensors supplied by D-M-E Company, Madison Heights, Mich. Of generally rectangular parallelepiped form these sensors require that the full width channel to the edge of the ejector plate be straight. Button mold pressure sensors are exemplified by D-M-E Standard BS-412C and BS-413C sensors. Of squat cylindrical form these sensors cannot be removed without disassembly of the ejector plate from the ejector retainer plate and are thus an integral part of the mold assembly.
Button sensors typically have exposed cables. The entrance of the cable, at the side of the button sensor is typically weak and subject to failure when relatively low forces are applied to the cable during installation or removal from the mold assembly. Cable failure at the connector end of the cable is also common due to forces applied to the cable during installation.
On both button and slide sensors, cable damage is the largest cause of sensor failure in the industry. Slide sensors generally have a Teflon or polyvinyl chloride coated cable enclosed in a flexible stainless steel armored jacket. The armored jacket protects the cable from abrasion but does not provide a stiff structural member for insertion or removal of the sensor from the slot or channel. The armored jacket also has relatively low transverse strength. Thus, the armored cable can be easily pinched and damaged. When such damage occurs, field repair is not possible because repair of the cable is difficult at best.
The cables above, whether or not armored, are not structurally rigid and the mold builder or assembler must provide positive means to hold the sensors in place. The friction spring on top of a commercially available slide sensor is normally not sufficient to properly retain the sensor in position in a production mold unless the sensor is installed from the top of a mold so that the force of gravity can provide additional holding force.
Button sensors are retained in place by the pockets for the sensors and therefore do not normally require restraining devices. However, many failures occur because the cables become displaced during assembly of the ejector and ejector retainer plates. The cables tend to migrate out of the slots between the plates and become pinched.
Slide sensors are located by the sidewalls of the slot or channel and the end of the channel just beyond the ejector pin. The slide sensors normally come in two standard configurations, the difference being the distance between the end of the slide sensor and the center line of the sensing element atop the sensor. The first standard with the distance larger by one-eighth inches was developed by applicant to provide greater protection to the sensor during insertion. However, molds provided with the more recent shorter standard can not use the longer sensors thus preventing interchangeability in the field.